The present invention relates to the field of microcontroller and more specifically microcontrollers used in furnace control systems.
Microcontrollers were widely used in heating and cooling control systems. In a furnace system, a microcontroller was responsible for operating, for example, valves, fans and igniters. The microcontroller also received inputs such as signals from limit switches. Due to the explosive nature of the gas used in furnaces, proper operation of the microcontroller was vital to insure safe operation of the furnace. Usually, in order to perform these functions, the microcontroller system included Read Only Memory (ROM) and Random Access Memory (RAM).
The microcontroller typically operated according to the cycle indicated in FIG. 1. Once started at box A, the microcontroller went through an initialization process at box B. Interrupts were enabled at box C. Then, ROM was tested at box D.
After the ROM test, the microcontroller would go into an operational loop. At box E, inputs were received. At box F, control functions were performed. At box G, outputs from the microcontroller were checked. At box H, RAM was tested. Lastly, at box I, the timers were checked to insure a constant time base signal was being generated. Then, the cycle would start over again at box E. The microcontroller also included a reset function (not shown) which causes the microcontroller to re-initialize when a signal at the reset port drops below a threshold voltage.
It was important that the microcontroller operate correctly and produce the proper outputs to insure safe operation of the furnace. Microcontrollers required some source of power for operation. One problem which commonly caused improper operation of the microcontroller was a drop in the microcontroller's power supply.
Another problem in the operation of the microcontrollers is that occasionally, the microcontroller may stop inadvertently at one step of its cycle and fail to proceed any further. This in turn affects the frequency and the repetition rate of the output signal
Thus it is an objective of the present invention to provide a microcontroller monitor which resets the microcontroller when the power supply drops below a predetermined minimum voltage. It is a further objective of the present invention to provide a microcontroller monitor which resets the microcontroller when the microcontroller gets stopped inadvertently as it proceeds through its operational loop.